1. Field of the Invention
The present invention relates to a spark plug which has an improved service life and is suitably used in an automobile combustion engine.
2. Description of the Related Art
Japanese Unexamined Patent Publication (Kokai) No. 5-343156 proposed a Pt-coated Ir material composed of an Ir wire having a diameter of 0.8 mm and having a circumferential surface coated with a 0.08 mm thick Pt coating. The Pt-coated Ir material is produced by melting an Ir powder, casting the melt to form an ingot, hot-working the ingot to an elongated wire having a diameter of 4 mm, inserting the wire in a Pt pipe and heating the assembly to form a Pt coating over the circumferential surface of the wire.
The proposed Pt-coated Ir material prevents consumption of Ir due to oxidation evaporation at elevated temperatures (hereinafter referred to as "oxidation consumption") and is advantageously disposed at the tip of a center electrode of a spark plug to provide an improved service life of the spark plug.
However, the Pt-coated Ir material has a problem in that the production process is complicated and the production cost is inevitably increased because of the necessity of an additional step of forming the Pt coating and the necessity of an expensive Pt pipe.